In optical double-star networks formed with passive fiber branchers, wherein every decentralized telecommunication equipment receives the downstream information transmitted by the central telecommunication equipment and only sorts the information intended for it out of this downstream information for the purpose of forwarding to the connected subscriber or subscribers, there is the possibility of an access to information that is intended for other subscribers who are connected to other decentralized telecommunication equipment.
For a telecommunication system wherein a plurality of subscriber locations are connected with light wave guide subscriber lines via one or more optical brancher equipment to a light wave guide link connected to a switching center, whereby one or more subscriber locations can be connected to an optical brancher means, it has already been disclosed (see German reference DE-C2-39 25 605) to counter this that a respective circuit is provided in the subscriber locations that initiates a coding key during the call set up between the subscriber location and the switching center. The coding key is transmitted to the switching center, whereupon the digital signals to be transmitted from the switching center to the subscriber location are transmitted in coded form using this coding key. This known solution, which also functions in the transmission of ATM cells, does not (in that the key to be employed is transmitted from the decentralized telecommunication equipment to the centralized equipment at the beginning of the transmission) preclude with certainty that an unauthorized person who obtained access to this key either intentionally or unintentionally (even if only because of a malfunction of the electronic equipment in the network), can thus also decode the encoded downstream signals. Furthermore, the risk of an unauthorized decoding also increases when a connection exists for a long time (such as, for example, a point-to-point dedicated line), since the unauthorized person has adequate time to effect the decoding.
A more effective protection of a passive optical telecommunication system against unauthorized access onto the digital signals transmitted therein is provided by a method for data securing as disclosed in German reference DE-C1-42 04 461 for a telecommunication system having a central telecommunication equipment and a plurality of decentralized telecommunication equipment. Each of the equipment is respectively connected via its own light wave guide link to an optical brancher that is connected either directly or via at least one further optical brancher to a common light wave guide terminal of the central telecommunication equipment via a light wave guide bus. The signal transmission proceeding from the central telecommunication equipment to the decentralized equipment is undertaken in a multiplex frame or in an ATM cell stream. The signal transmission proceeding from the decentralized telecommunication equipment to the centralized equipment is respectively undertaken in a time channel of the multiplex frame allocated to the respective decentralized equipment, preferably upon adaptive control of the time slot of the time channel, proceeding in a TDMA method or, respectively, with ATM cells in a TDMA method. According to this method, a public key is calculated from a secret key prescribed in a decentralized telecommunication equipment, this public key being communicated in an upstream direction to the central telecommunication equipment where it is used for encoding (secondary key) information for the adaptation of the decentralized equipment to repeatedly modified transmission modalities that are individually provided in the central equipment for this decentralized equipment.